There is a known technology which detects positions of a human face or eyes in an image including the human face. For example, a face detection apparatus is disclosed in JP 2001-216515A. The apparatus divides an input image into sub-regions to detect a candidate region for eyes and a candidate region for an area between the eyes by utilizing the luminance feature of each sub-region. Then, the area between the eyes is determined by the positional relationship of the candidate region for the eyes and the candidate region for the area between the eyes to determine the face and eye regions so as to include the detected region of the area between the eyes.
However, the face detection apparatus disclosed in JP 2001-216515A detects a narrow region such as the area between the eyes in a wide face region, therefore leading to a large load imposed during the image processing. Further, the apparatus judges whether a subject wears a pair of glasses based on a gray level histogram or edge information of the sub regions which are adjacent vertically and horizontally. Thus, the determination of the presence of the glasses may be subject to influences of the frame design of the glasses. Since the apparatus identifies the positions of the left and right eyes based on the region of the area between the eyes, which is detected under the above-described conditions, the possibility of the error detection is not eliminated.
Further, an open and closed state detection apparatus is disclosed in JP 2004-220080A. The apparatus detects the open and closed state of a driver's eye who operates a movable body such as a vehicle. The open and closed state detection apparatus disclosed in JP 2004-220080A captures an image of a subject's face to detect an (image of) upper eyelid in the obtained face image, and then the apparatus approximates the detected eyelid by a multidimensional curve to calculate the curvature of the approximated multidimensional curve. The calculated curvature is set as a curvature of the upper eyelid, and the state of the eye is judged to be open or closed based on how large (or small) the curvature is.
In the above-described open and closed detection technology, when detecting the upper eyelid in the face image, the error detection of the upper eyelid may occur due to influence of ambient light and the glasses. In addition, it is necessary to approximate the eyelid by the multidimensional curve and calculate the curvature of the approximated multidimensional curve in each upper eyelid detection process. Hence, an enormous amount of processing operation is required.
A need exists for an eyelid detection apparatus which is not susceptible to the drawback mentioned above.